Disk drives are a common type of magnetic data storage device. Digital information is stored within concentric tracks on a storage disk which is coated with a magnetic material that is capable of changing its magnetic orientation in response to an applied magnetic field.
During operation of a conventional disk drive, the disk is rotated about a central axis at a substantially constant rate. To read data from or write data onto the disk, a head is held close to a desired track of the disk while the disk is spinning. Writing is performed by delivering a write signal having a variable current to the head while the head is held close to the desired track. The write signal creates a variable magnetic field at a gap portion of the head that induces magnetic polarity transitions into the desired track which represent the data being stored.
Reading is performed by sensing the magnetic polarity transitions on the rotating track with the head. As the disk spins relative to the head, the magnetic polarity transitions on the track present a varying magnetic field to the head. The head converts the varying magnetic field into an analog read signal that is then processed by a read channel circuit. The read channel circuit converts the analog read signal into a timed digital signal that is processed by a data controller to extract data for communication to a host computer system.
The head can include a single element, such as an inductive read/write element for use in both reading and writing, or it can include separate read and write elements. Heads that include separate elements for reading and writing are known as “dual element heads” and usually include a magneto-resistive (MR) read element for performing the read function.
Dual element heads can have more sensitivity than single element heads because the dual elements can be separately optimized to perform particular functions (i.e., reading data or writing data). For example, MR read elements are generally more sensitive to small variable magnetic fields than are single element inductive heads and, thus, can read much fainter signals from the disk surface. The higher sensitivity of MR elements may allow data to be more densely packed on the surface without an unacceptable loss of read performance.
MR read elements generally include a strip of MR material that is held between two magnetic shields. The resistance of the MR material varies almost linearly with the applied magnetic field. During a read operation, the MR strip is held near a desired track to sense the varying magnetic field caused by the magnetic transitions on the track. A constant DC current is passed through the MR strip while the varying magnetic fields from the track cause a variable voltage to be generated across the MR strip. By Ohm's law (i.e., V=IR), the variable voltage is proportional to the varying resistance of the MR strip and, hence, is representative of the data stored within the desired track. The variable voltage signal (which is the analog read signal) is then processed by the read channel circuit and data controller to extract data for use by the host.
There are many operational variables that can adversely affect the read performance of a magnetic disk drive. Among the variables, those which cause temperature variations in the MR element can be particularly troublesome. More specifically, because MR elements are positive temperature coefficient devices, increase in the temperature of an MR element causes an increase in the resistance of the MR element. Because the read signal (in volts) is proportional to the variations in resistance of the MR element multiplied by the bias current, and because the bias current is usually a constant DC current, when the temperature of the MR element is increased, a thermal component is generated which adds to the value of the read signal. Accordingly, the read signal has a data component and a thermal component.
One operational variable is related to foreign particles or other aberrations on the surface of the disk that can contribute to generation of the thermal component of the read signal. These foreign particles and aberrations are known as asperities. Collisions between the asperities and the head cause the head to be further heated. This increase in head temperature increases the resistance of the MR element. Because the bias current is usually constant, the resulting voltage appears to be greater than the voltage that should be present based upon the data stored on the magnetic disk. The additive signal resulting from the change in temperature of the MR element is known as a thermal asperity.
Another operational variable is related to variations in the gap between the head and the disk due to the disk surface variations, which can also contribute to generation of the thermal component of the read signal. The head is heated to a temperature above the ambient temperature (e.g., 20 degrees above ambient temperature) by the bias current. The temperature of the disk is essentially equal to the ambient temperature and thereby operates as a heat sink which takes heat away from the head. As the relative gap varies due to the surface variations of the disk, the head is correspondingly cooled at varying rates, which causes a variation in the resistance of the head MR element. This resistance variation contributes to the thermal component of the read signal, and is known as baseline modulation.
By way of example, when a disk protrusion causes the gap between the disk and the MR element to decrease, the disk conducts heat from the MR element at a higher rate which causes the temperature of the MR element to rapidly decrease. As the temperature of the MR element decreases, the resistance of the MR element likewise decreases. Similarly, when a valley on the disk increases the gap between the disk and the MR element, the disk conducts head from the MR element at a lower rate which causes the temperature of the MR element to rapidly increase. As the temperature of the MR element increases, the resistance of the MR element likewise increases. Accordingly, the magnitude of the thermal component of the read signal from the MR element can be modulated by the surface roughness of the disk.
As explained above, the thermal component of the read signal due to change in temperature of the head is known as a thermal asperity. When the level of this thermal component becomes excessive, the head is considered to be subjected to a thermal asperity condition. Some disk drives include a thermal asperity detector which is configured to detect the presence of a thermal asperity condition so that effects of the condition on the read signal may be suppressed. FIG. 1 is a block diagram of a conventional thermal asperity detector 100. The detector 100 includes an input network 110, a variable gain amplifier (VGA) 120, a low pass filter (LPF) 130, and a comparator 140. An analog read signal is passed through the input network 110, which may be configured as an AC coupling circuit. The VGA 120 amplifies the read signal from the input network 110. The LPF 130 filters the amplified read signal to extract the DC component. The comparator 140 compares the filtered read signal to a threshold value and, when the filtered read signal exceeds the threshold value 150, sets a thermal asperity flag to indicate the occurrence of a thermal asperity condition of the head.
The threshold value 150 is typically set to a defined margin level above the expected envelope of the filtered read signal from the LPF 130 in an attempt to avoid false indications of the presence of a thermal asperity condition of the head. FIG. 2 shows graphs that illustrate a filtered read signal, the threshold value, and the varying states of the thermal asperity flag. The filtered read signal from the LPF 130 is represented by waveform 200, the threshold value is represented by line 210, and the thermal asperity flag is represented by waveform 220. The thermal asperity flag (waveform 220) is set to logic 1 while the filtered read signal (waveform 200) exceeds the threshold value (line 210). Accordingly, selecting a low threshold value can increase the sensitivity of the thermal asperity detector to detecting thermal asperity conditions, but it can also increase the occurrence of false indications of thermal asperity conditions when an insufficient margin is maintained between the data component of the filtered read signal and the threshold value.
Complicating the selection of an appropriate threshold value is that the average DC value of the read signal can vary over time due to, for example, baseline wander. FIG. 3 is a graph that illustrates exemplary baseline wander in a read signal following reading of a long DC pattern. Because baseline wander increases the expected magnitude range of the read signal, a sufficient margin should be maintained between the selected threshold value and the data component of the read signal. Disk drives that utilize perpendicular recording can have a much greater baseline wander in read signals than disk drives that utilize longitudinal recording. Greater baseline wander may necessitate a greater margin which, correspondingly, may reduce the ability of a thermal asperity detector to detect small to medium indications of a thermal asperity condition of a head. Accordingly, there is a need to develop further methods and apparatus capable of improving the detection of a thermal asperity condition.